What purpose do oxygen sensors serve in an automobile engine?

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An oxygen sensor measures the air-to-gasoline ratio of a vehicle's emissions and provides feedback to the engine-management computer. This data feed allows the engine to run efficiently and produce a minimal amount of emissions. A perfect air-to-fuel ratio achieves ideal operating conditions.

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A gasoline engine uses an electrical spark to ignite a mixture of gas and air. The resulting combustion creates motion in the moving parts of the engine, ultimately producing movement of the vehicle. If this mixture has less air than the optimal ratio, the vehicle runs rich and emits unburned fuel. This unburned gasoline decreases fuel efficiency and increases carbon-based pollutants in the vehicle's emissions. If the mixture has too much air than the optimal ratio, the vehicle runs lean and emits excess oxygen. This extra oxygen causes poor engine performance and creates nitrogen-oxide pollutants in the vehicle's emissions.

The oxygen or O2 sensor is inside the vehicle's emissions system. Most oxygen sensors generate a chemical reaction in the presence of engine emissions and emit varying voltage levels depending on the richness or leanness of the exhaust. The engine-management computer reads these voltage patterns and adjusts the vehicle's air-to-fuel ratio accordingly, maintaining an optimal mixture and perfect engine performance. If an oxygen sensor fails, the computer no longer has feedback and the engine performs poorly.